The present invention relates to a method for producing a cordierite body, particularly to a cordierite body to be used as the material of the parts subjected to repeated cycles of rapid increases and decreases in temperature.
Cordierite (2MgO.2Al.sub.2 O.5SiO.sub.2) has an excellent low thermal expansion property. Therefore, cordierite is used as the material of parts requiring thermal shock resistance in order to endure the repeated cycles of rapid increases and decreases in temperature, such as a catalyst support for purifying exhaust gas and a porcelain insulator.
The coefficient of thermal expansion of cordierite has been generally known as 26.0.times.10.sup.-7 /.degree.C. in the range of 25.degree. to 1000.degree. C.
Recently, ceramic having more excellent thermal shock resistance has been required. Upon making various studies, a cordierite body having excellent low expansion property as low as 17.0.times.10.sup.-7 /.degree.C. (25.degree. to 1000.degree. C.) can be obtained by eliminating alkali metals such as natrium and kalium, alkaline-earth metals such as calcium and other impurities having an adverse effect on the low thermal expansion property of cordierite, from the material.
Furthermore, it has been reported that by orienting the anisotropic property of the cordierite crystals, cordierite bodies having coefficients of thermal expansion of less than 11.0.times.10.sup.-7 /.degree.C. in the range of 25.degree. to 1000.degree. C. in at least one direction can be obtained.
For example, one of the methods for orienting the cordierite crystals to reduce the thermal expansion of the cordierite body in at least one direction is shown in U.S. Pat. No. 3,885,977.
In U.S. Pat. No. 3,885,977, there is stated that a cordierite body having thermal expansion as low as 11.0.times.10.sup.-7 /.degree.C. in a specific direction is obtained by forming a batch raw material containing plate-shaped clay particles or stacked clay which can be delaminated into plate-shaped particles during processing by such means as to impart a planar orientation to the plate-shaped clay particles, for example, extrusion forming, drying and firing the obtained formed body.
As described above, by anisostatically forming a batch raw material mainly composed of plate-shaped particles of kaolin mineral and talc, a planar orientation is imparted to the plate-shaped particles, and the obtained cordierite body exhibits a low expansion property in the direction along the extrusion direction.
However, coefficients of thermal expansion of the obtained cordierite body in the direction perpendicular to the extrusion direction and that of the thickness thereof remain larger than that in the extrusion direction thereof. The difference of the coefficients of thermal expansion becomes larger as the coefficient of thermal expansion in the extrusion direction becomes smaller.
When coefficients of thermal expansion in the above described directions are largely different from each other, the cordierite body is liable to be damaged due to thermal distortion when being subjected to rapid increases and decreases in temperature.
In the cordierite honeycomb structure which is formed by extruding, the difference between the coefficient of thermal expansion in the direction of wall thickness and that in the extruding direction is not a serious problem, since the wall thickness of the honeycomb structure is very thin and also between adjacent thin walls, open passages are formed.
However, large difference between the coefficient of thermal expansion in the extruding direction and that in the direction which is perpendicular to the extruding direction on the same plane thereas, causes the honeycomb structure to be damaged.
The cordierite body which is obtained by mixing and kneading the batch raw material containing plate-shaped particles, forming the batch raw material by anisostatic forming means such as extrusion forming and firing the formed body, exhibits an anisostatic property in its coefficient of thermal expansion.
Talc has been used as one of main ingredients of the batch raw material. When the talc is pulverized, it is usually delaminated along (001) plane into plate shaped particles. Therefore, the cordierite body which is obtained by extruding the batch raw material containing talc particles also exhibits anisostatic property in the coefficient of thermal expansion.
One object of the present invention is to provide a method for producing a cordierite body having low coefficient of thermal expansion in any direction.
Another object of the present invention is to provide a method for producing a cordierite body which does not shrink during the firing step.
Still another object of the present invention is to provide a method for producing a product made of cordierite, which is required to have such thermal shock resistance as to endure the repeated cycles of rapid increase and decrease in temperature, for example, a catalyst support for purifying exhaust gases, and a porcelain insulator.
A further object of the present invention is to provide a cordierite body having isotropic thermal expansion property and low coefficient of thermal expansion by means of anisostatic forming.